Method for detecting and treating ventricular arrhythmia

ABSTRACT

A system and method for long-term monitoring of cardiac conditions such as arrhythmias is disclosed. The invention includes a pulse generator including means for sensing an arrhythmia. The pulse generator is coupled to at least one subcutaneous electrode or electrode array for providing electrical stimulation such as cardioversion/defibrillation shocks and/or pacing pulses. The electrical stimulation may be provided between multiple subcutaneous electrodes, or between one or more such electrodes and the housing of the pulse generator. In one embodiment, the pulse generator includes one or more electrodes that are isolated from the can. These electrodes may be used to sense cardiac signals.

This application is a continuation of U.S. application Ser. No.14/275,845, filed May 12, 2014, which is a continuation of U.S.application Ser. No. 10/968,889, filed Oct. 21, 2004, which is acontinuation of U.S. application Ser. No. 10/460,300, filed Jun. 13,2003 (now abandoned), which is a continuation of U.S. application Ser.No. 09/990,045, filed Nov. 21, 2001 (now abandoned), which claims thebenefit of U.S. Provisional Application No. 60/252,811, filed Nov. 22,2000.

U.S. application Ser. No. 14/275,845 is also a continuation of U.S.application Ser. No. 13/476,940, filed May 21, 2012. U.S. applicationSer. No. 13/476,940 is a continuation of U.S. application Ser. No.11/981,410, filed Oct. 31, 2007 (now abandoned). U.S. application Ser.No. 11/981,410 is a continuation of U.S. application Ser. No.10/949,877, filed Sep. 24, 2004 (now abandoned), which is a continuationof U.S. application Ser. No. 09/990,045, filed Nov. 21, 2001 (nowabandoned), which claims the benefit of U.S. Provisional Application No.60/252,811, filed Nov. 22, 2000. U.S. application Ser. No. 11/981,410 isalso a continuation of U.S. application Ser. No. 10/968,889.

This application is also a continuation of U.S. application Ser. No.13/476,940, filed May 21, 2012.

The entire contents of each of the aforementioned U.S. applications isincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for treatingventricular arrhythmias; and more particularly, relates to a method andapparatus for long-term monitoring of arrhythmias, and for the deliveryof acute tachyarrhythmia and bradyarrhythmia therapy using asubcutaneous stimulation device.

DESCRIPTION OF THE PRIOR ART

It has long been known to use implantable systems to protect patientsthat are at risk for life-threatening arrhythmias. For example, rapidheart rhythms commonly referred to as tachyarrhythmias are generallytreated using implantable devices such as the Medtronic Model 7273 GEMII DR or the 7229 GEM II SR, both commercially available from theMedtronic Corporation. These systems detect the presence oftachyarrhythmia conditions by monitoring the electrical and mechanicalheart activity (such as intra-myocardial pressure, blood pressure,impedance, stroke volume or heart movement) and/or the rate of theelectrocardiogram. These devices require that one or more defibrillationelectrodes be positioned within the atrium and/or ventricle of apatient's heart using current endocardial lead placement techniques. Theuse of such systems provides consistent long-term monitoringcapabilities, and relatively good protection against life-threateningtachyarrhythmias.

Similarly, bradyarrhythmias, which are heart rhythms that are too slow,are generally treated using implantable pulse generators. Such devicesare described in U.S. Pat. Nos. 5,158,078, 4,958,632, and 5,318,593, forexample. As with devices to treat tachyarrhythmias, most implantablepulse generators that treat these types of conditions generally requireleads that are implanted within one or more cardiac chambers.

Although the use of endocardial leads placed within the cardiac chambersof a patient's heart provides the capability to deliver a relativelyreliable, long-term arrhythmia therapy, there are disadvantagesassociated with such treatments. The placement of these leads requires arelatively time-consuming, costly procedure that is not without risks tothe patient including infection, the possibility of vascularperforation, and tamponade.

Moreover, some people are not candidates for endocardial leads. Forexample, patients with artificial mechanical tricuspid valves aregenerally not candidates for leads that extend from the right atrium,through this valve, to the right ventricle, as is the case with mostright ventricular endocardial leads. This is because the use of suchleads interfere with the proper mechanical functioning of the valves.Other patients that are not candidates for endocardial lead placementinclude those with occluded venous access, or patients with congenitalheart defects. Patients that are contraindicated for endocardial leadplacement must often undergo a procedure to attach the lead to theexternal surface of the heart. This type of epicardial lead placementinvolves a more invasive procedure that requires a longer recovery time,makes follow-up procedures very difficult, and is also associated withincreased patient risk, including an increased chance of contracting aninfection.

Another problem associated with both endocardial and epicardial leadsinvolves patient growth. More specifically, a lead placed within achild's cardiac vasculature will likely need to be re-positioned orreplaced as the child matures. Such lead replacement procedures can bedangerous, especially when previously-placed leads are extracted ratherthan left in position within the body.

One alternative to endocardial and epicardial leads involvessubcutaneously-placed electrode systems. For example, in U.S. Pat. No.RE27,652 by Mirowski, et al., a defibrillation system employs aventricular endocardial electrode and a plate electrode mounted to theheart directly, subcutaneously, or to the skin to deliver high-voltagetherapy to the patient. A similar lead system disclosed in U.S. Pat. No.5,314,430 to Bardy includes a coronary sinus/great vein electrode and asubcutaneous plate electrode located in the left pectoral region whichmay optionally take the form of a surface of the defibrillator housing.

What is needed, therefore, is a system and method that can providelong-term monitoring for various types of arrhythmias, provide patienttherapy when needed, and also overcome the problems associated with bothendocardial and epicardial lead placement.

SUMMARY OF THE INVENTION

The current invention provides a system and method for long-termmonitoring for arrhythmias. The invention includes a pulse generatorincluding means for sensing an arrhythmia. The pulse generator iscoupled to at least one electrode or electrode array for providingelectrical stimulation to a patient. The stimulation may includecardioversion/defibrillation shocks and/or pacing pulses. The electricalstimulation may be provided between multiple electrodes, or between oneor more electrodes and the housing of the pulse generator. In oneembodiment, the pulse generator includes one or more electrodes that areisolated from the can. These electrodes may be used to sense cardiacsignals.

According to one embodiment of the invention, an apparatus is providedfor monitoring cardiac signals of a patient. The apparatus includes ahermetically-sealed housing, sensing means included within the housing,and first and second electrode sets coupled to the sensing means. Thefirst electrode set includes at least one electrode adjacent to asurface of the housing positionable proximate subcutaneous tissue at afirst location in the patient's body. The second electrode set iscoupled to a connector on the housing and forms an electrode arraysubcutaneously-positionable in the patient's body at a locationdifferent from the first location.

According to another embodiment of the invention, a method of therapy isprovided. This method includes monitoring the patient's cardiac signalsfor a condition such as an arrhythmia, and thereafter delivering aelectrical therapy to a patient via a subcutaneous electrode array isthe condition is detected. Other aspects of the invention will becomeapparent from the drawings and the accompanying description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary subcutaneous electrode and pulsegenerator as may be used in accordance with the current invention.

FIG. 2 is a block functional diagram of an illustrative embodiment of apulse generator that may be employed according to the present invention.

FIG. 3A is a top view of an electrode array 300 as may be used with thecurrent invention.

FIG. 3B is a top view of an electrode array, according to anotherembodiment of the present disclosure.

FIG. 4A is a side view of a pulse generator illustrating the orientationof electrodes A, B and C disposed on the device housing.

FIG. 4B is a side view of a pulse generator wherein at least one of theelectrodes extends away from the pulse generator via a lead extension.

FIG. 4C is a side view of a pulse generator wherein at least one of theelectrodes is located at a proximal end of a lead.

FIG. 4D is a side view of a pulse generator wherein multiple electrodesare located on an edge of a device housing.

FIG. 4E is a side view of yet another embodiment of a device housingincluding an array of electrodes.

FIG. 4F is a side view of a device having a first alternative shape.

FIG. 4G is a side view of a device having a second alternative shape.

FIG. 5 is a timing diagram illustrating one embodiment of a detectionmethod used during bradyarrhythmia monitoring.

FIG. 6 is a block diagram illustrating an electrode array positionedaround a patient's side, with electrode coils extending to the patient'sback.

FIG. 7 is a block diagram illustrating an electrode array positioned onpatient's back in a more superior position.

FIG. 8 is a block diagram illustrating an electrode array positionedaround a patient's side, with coil electrodes extending to the patient'sback in a more posterior position.

FIG. 9 is a block diagram illustrating an electrode array positioned ona patient's back, and a second subcutaneous disk electrode positioned ona patient's chest.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The current invention provides a system and method for long-termmonitoring for arrhythmias. The invention also provides acute therapydelivery in the event an arrhythmia episode is detected. According toone embodiment of the invention, a subcutaneous pulse generator isprovided. This pulse generator may be a transthoracic ImplantableCardioversion/Defibrillator (ICD) such as the GemDR™ Model 7271 or theGEM II VR Model 7229, both commercially available from the MedtronicCorporation. The pulse generator is coupled to at least onesubcutaneously-placed electrode or electrode array.Cardioversion/defibrillation pulses and/or pacing pulses may bedelivered between the electrode and the can of the device, or betweentwo subcutaneously-placed electrodes.

FIG. 1 illustrates an implantable pulse generator 10 and an exemplaryassociated lead system according to the current invention. Pulsegenerator 10 includes a device housing 12, and is further coupled to alead 14 which may be implanted subcutaneously in the left chest or onthe back as discussed below. Lead 14 may include a subcutaneous plateelectrode 16, which may be any of the various known subcutaneous plateelectrodes. This type of subcutaneous electrode may be located proximalthe left ventricular-cavity on the patient's 30 chest, on the patient'sside or back, or any other portion of the body appropriate for providingelectrical stimulation to the heart. Similar electrodes are disclosed inU.S. Pat. Nos. 4,932,407, 5,261,400, and 5,292,338, all incorporatedherein by reference. During use, electrical stimulation may be deliveredto heart 18 between electrode 16 and device housing 12.

FIG. 2 is a block functional diagram of an illustrative embodiment of apulse generator that may be employed according to the present invention.As illustrated, the device is embodied as a microprocessor-basedstimulator. However, other digital circuitry embodiments and analogcircuitry embodiments are also believed to be within the scope of theinvention. For example, devices having general structures as illustratedin U.S. Pat. No. 5,251,624 issued to Bocek et al., U.S. Pat. No.5,209,229 issued to Gilli, U.S. Pat. No. 4,407,288, issued to Langer etal, U.S. Pat. No. 5,662,688, issued to Haefner et al., U.S. Pat. No.5,855,593, issued to Olson et al., U.S. Pat. No. 4,821,723, issued toBaker et al. or U.S. Pat. No. 4,967,747, issued to Carroll et al., allincorporated herein by reference in their entireties, may also beusefully employed in conjunction with the present invention. FIG. 1should thus be considered illustrative, rather than limiting with regardto the scope of the invention.

The primary elements of the apparatus illustrated in FIG. 2 are amicroprocessor 100, read-only memory (ROM) 102, random-access memory(RAM) 104, a digital controller 106, an input amplifier circuit 110, twooutput circuits 108 and 109, and a telemetry/programming unit 120.Read-only memory stores software and/or firmware for the device,including the primary instruction set defining the computationsperformed to derive the various timing intervals employed by the device.RAM 104 generally serves to store variable control parameters, such asprogrammed pacing rate, programmed cardioversion/defibrillationintervals, pulse widths, pulse amplitudes, and so forth which areprogrammed into the device by the physician. Random-access memory 104also stores derived values, such as the stored time intervals separatingtachyarrhythmia pulses and the corresponding high-rate pacing interval.

Controller 106 performs all of the basic control and timing functions ofthe device. Controller 106 includes at least one programmable timingcounter, which is used to measure timing intervals within the context ofthe current invention. On time-out of the pacing escape interval or inresponse to a determination that a cardioversion, defibrillation, orpacing pulse is to be delivered, controller 106 triggers the appropriateoutput pulse from high-voltage output stage 108, as discussed below. Inone embodiment, controller may also control the amplitude of pacingpulses, as well as the energy associated with defibrillation andcardioversion shocks.

Following generation of stimulus pulses, controller 106 may be utilizedto generate corresponding interrupts on control lines 132 tomicroprocessor 100, allowing it to perform any required mathematicalcalculations, including all operations associated with evaluation ofreturn cycle times and selection of anti-tachyarrhythmia therapiesaccording to the present invention. The timing/counter circuit incontroller 106 also may control timing intervals such as ventricularrefractory periods, as is known in the art. The time intervals may bedetermined by programmable values stored in RAM 104, or values stored inROM.

Controller 106 may also generate interrupts for microprocessor 100 onthe occurrence of sensed ventricular depolarizations or beats. Thetiming and morphology of sensed cardiac waveforms may also be used bymicroprocessor 100 to determine whether an arrhythmia is occurring sothat therapy may be delivered as discussed further below.

Output stage 108 contains a high-output pulse generator capable ofgenerating cardioversion/defibrillation pulses. According to the currentinvention, these pulses may be applied between a subcutaneous electrodeor electrode array coupled to terminal 134 and the can of the pulsegenerator. Alternatively, the pulses may be provided between anelectrode coupled to terminal 134 and a second subcutaneous electrode orelectrode array coupled to terminal 136. Typically the high-output pulsegenerator includes one or more high-voltage capacitors, a chargingcircuit, and a set of switches to allow delivery of monophasic orbiphasic cardioversion or defibrillation pulses to the electrodesemployed. Output circuit 108 may further provide pacing pulses to theheart under the control of controller 106. These pacing pulses, whichmay be between 50 and 150 volts in amplitude, are provided via one or 25more of the subcutaneously-located electrodes.

Sensing of ventricular depolarizations (beats) is accomplished by inputcircuit 110, which is coupled to electrode 138 and one of electrodes 140and 142. This circuitry may include amplification, and noise detectionand protection circuitry. In one embodiment, signal sensing is disabledduring periods of excessive noise. Noise rejection filters and similarcircuitry may also be included, as is known in the art. Input circuit110 provides signals indicating both the occurrence of naturalventricular beats and paced ventricular beats to the controller 106 viasignal lines 128. Controller 106 provides signals indicative of theoccurrence of such ventricular beats to microprocessor 100 via signallines 132, which may be in the form of interrupts. This allows themicroprocessor to perform any necessary calculations or to update valuesstored in RAM 104.

Optionally included in the device may be one or more subcutaneously orcutaneously-positioned physiologic sensors 148, which may be any of thevarious known sensors for use in conjunction with implantablestimulators. Any sensor of this type known in the art may be employedwithin the context of the current invention. Additionally, if desired,sensors positioned within the cardiovascular system may be utilized. Forexample, sensor 148 may be a hemodynamic sensor such as an impedancesensor as disclosed in U.S. Pat. No. 4,865,036, issued to Chirife or apressure sensor as disclosed in U.S. Pat. No. 5,330,505, issued toCohen, both of which are incorporated herein by reference in theirentireties. Alternatively, sensor 148 may be a demand sensor formeasuring cardiac output parameters, such as an oxygen saturation sensordisclosed in U.S. Pat. No. 5,176,137, issued to Erickson et al. or aphysical activity sensor as disclosed in U.S. Pat. No. 4,428,378, issuedto Anderson et al., both of which are incorporated herein by referencein their entireties.

Sensor processing circuitry 146 transforms the sensor output intodigitized values for use in conjunction with detection and treatment ofarrhythmias. These digitized signals may be monitored by controller 106and microprocessor 100 and used alone or in combination with sensedelectrical cardiac signals to provide diagnostic information used todetermine the onset of an arrhythmia or other cardiac conditions. Thesesignals may also be used to determine an optimal time for shockdelivery. For example, an impedance sensor may be used to determine whena patient has exhaled so that shock delivery may occur when the lungsare relatively deflated, since this may result in lower defibrillationthresholds (DFTs). Sensor signals may also be stored in RAM 104 forlater diagnostic use.

External control of the implanted cardioverter/defibrillator isaccomplished via telemetry/control block 120 that controls communicationbetween the implanted cardioverter/pacemaker and an external device 121.Any conventional programming/telemetry circuitry is believed workable inthe context of the present invention. Information may be provided to thecardioverter/pacemaker from the external device and passed to controller106 via control lines 130. Similarly, information from thecardioverter/pacemaker may be provided to the telemetry block 120 viacontrol lines 130 and thereafter transferred to the external device.

In one embodiment, the external device 121 is a programmer that may beutilized to diagnose patient conditions and to provide any necessaryre-programming functions. In another embodiment, the external device maybe a patient interface used to provide information to, and/or receivecommands from, the patient. For example, the patient interface may be anexternally-worn device such as a wrist band that provides a warning to apatient concerning an impending shock. The patient may be allowed tocancel the shock if the patient believes the shock was prescribederroneously. This may be accomplished, for example, by pushing a button,or issuing a voice command. The patient interface may provide additionalinformation, including a warning that medical attention is required,and/or an indication concerning a low power source. If desired, thepatient interface could automatically place an emergency telephone callvia a wireless link, and/or could issue patient positional informationvia a global positioning system (GPS).

Any other system and method used for the detection and treatment oftachyarrhythmias may be incorporated within the current invention. Suchsystems and methods are described in U.S. Pat. Nos. 5,849,031,5,193,535, and 5,224,475. In one embodiment the system may include“tiered therapies” for delivering treatment based on the type ofarrhythmia detected by the device. According to this approach,arrhythmias are differentiated by analyzing the rate and morphology of asensed cardiac signal. Those arrhythmias considered less dangerous suchas ventricular tachycardias (VTs) may be treated by delivering a seriesof low-power, relatively high-rate, pacing pulses to the heart. Thistherapy is often referred to as anti-tachyarrhythmia pacing therapy(ATP). In contrast, more perilous arrhythmias such as ventricularfibrillations (VFs) may be treated by immediately delivering moreaggressive shock therapy. This type of system is described in U.S. Pat.No. 5,193,536, issued to Mehra, U.S. Pat. No. 5,458,619 to Olson, U.S.Pat. No. 6,167,308 to DeGroot, and U.S. Pat. No. 6,178,350 to Olson, etal., all incorporated herein by reference. Within the context of thecurrent invention, ATP therapy is delivered using one or moresubcutaneous electrodes in the manner discussed below. In one embodimentof the invention, a separate electrode may be provided within asubcutaneous electrode array for delivering the ATP therapy.

According to another aspect of the inventive system, the device mayinclude means for decreasing discomfort associated with high-voltageshocks. It is well known that high-voltage shocks are painful for thepatient. This discomfort can be minimized by decreasing the amount ofenergy associated with the shock. One mechanism for accomplishing thisinvolves delivering a pre-shock pulse waveform, as described in U.S.Pat. No. 5,366,485 issued to Kroll. In one embodiment, this type ofwaveform could be a programmable feature that is controlled bycontroller 106 via parameters stored in RAM 104.

In yet another embodiment of the invention, the implantable deviceincludes a drug pump 150 as shown in FIG. 2. This pump may be used todeliver a biologically-active agent such as an analgesic drug to thepatient prior to shock delivery to reduce discomfort. The drug deliverymay be accomplished via a catheter 152 that is implanted subcutaneouslyor within the patient's vascular system. A similar system is describedin U.S. Pat. No. 5,893,881 to Elsberry, incorporated herein byreference. Alternatively, or in addition, this pump may deliver an agentsuch as D-salotol, Procainamide or Quinidine to reduce thedefibrillation threshold of the required shock, thereby serving toreduce pain. In a more complex embodiment, two separate drug pumps mightbe employed to allow delivery of the threshold reducing agent alone orin conjunction with an analgesic.

Pain control may also be accomplished by providing spinal cordstimulation (SCS). For example, the Medtronic Itrel II implantableneurostimulation system is widely implanted for treatment andalleviation of intractable pain. Clinical reports and studies have shownthat SCS can reduce the discomfort associated with high-voltage shocks.This type of system may utilize a lead system of the type described inU.S. Pat. No. 5,119,832, 5,255,691 or 5,360,441. These leads, as well asthe Medtronic Model 3487A or 3888 leads, include a plurality of spacedapart distal electrodes that are adapted to be placed in the epiduralspace adjacent to spinal segments T1-T6 to provide SCS stimulation forpain reduction. In this embodiment, initial detection and verificationof fibrillation is followed by epidural neural stimulation to produceparaesthesia. Thereafter, a shock may be delivered. Should thecardioversion shock prove unsuccessful, the process is repeated untilthe cardioversion therapies prove successful or are exhausted. Whensuccessful defibrillation is confirmed, the epidural SCS stimulation ishalted.

In addition to SCS therapy, other types of stimulation such asTranscutaneous Neurological Stimulators (TENs) may be provided viaelectrode patches placed on the surface of a patient's body.Subcutaneously-placed electrodes may also be positioned in the T1-T6area or in other areas of the body to deliver subcutaneous electricalstimulation to reduce pain. In the context of the current invention, thesubcutaneously-placed electrode arrays may include specializedelectrodes to deliver the subcutaneous stimulation prior to shockdelivery to reduce patient discomfort.

Turning now to a more detailed discussion of the electrode systems usedwith the current invention, the electrode may be of a type shown inFIG. 1. Alternatively, this electrode array may be similar to the Model6996 SQ commercially-available from the Medtronic Corporation.

FIG. 3A is a top view of an electrode array 300 as may be used with thecurrent invention. Electrode array 300 is coupled to distal end of lead302. The array includes multiple finger-like structures 304A through304E. More or fewer of these finger-like structures may be provided.Each finger includes a defibrillation coil electrode shown as 306Athrough 306E. When connector 308 is coupled to a pulse generator, acardioversion/defibrillation pulse may be provided via one or more ofthe electrodes 306A through 306E. In one embodiment, the electrodes thatare activated may be selected via a switch provided by the lead.

Electrode array 300 may include one or more sensing electrodes such aselectrode 310 provided for sensing cardiac signals. This electrode maybe used in a unipolar mode wherein signals are sensed between anelectrode and the device housing. Alternatively, sensing may beperformed between electrode 310 and one of the coil electrodes 306 oranother sensing electrode.

In use, the fingers 304 of electrode array are positioned under the skinon a patient's chest, side, back, or any other point of the body asrequired. Insulative spacers may be located between the fingers, ifdesired, to prevent the coil electrodes 306A-E from shorting together.If desired, multiple such electrode arrays may be used in conjunctionwith the current invention. For example, one electrode array may bepositioned on the chest over the left ventricle, while another electrodearray is positioned behind the left ventricle on the back.Cardioversion/defibrillation shocks or pacing pulses may be deliveredbetween the two electrode arrays. Alternatively, electrical stimulationmay be provided between one or more electrode arrays and the devicehousing. As noted above, sensing of the patient's cardiac signals may beperformed between a subcutaneous electrode array and the device can.

FIG. 3B is a top view of an alternative embodiment of electrode array,shown as array 300A. In this embodiment, fingers 320A through 320C havea serpentine shape. More or fewer such fingers may be provided. Thisshaped array directs current provided by coiled electrodes 322A through322C through a larger tissue area, thereby decreasing defibrillationthresholds in some instances. This embodiment may also include one ormore sensing electrodes 322. Any other shape may be utilized for theelectrode array.

The electrodes used with the current invention may be any of theelectrode types now known or known in the future for subcutaneousdelivery of electrical stimulation. Such electrodes may be coated with abiologically-active agent such as glucocorticoids (e.g. dexamethasone,beclamethasone), heparin, hirudin, tocopherol, angiopeptin, aspirin, ACEinhibitors, growth factors, oligonucleotides, and, more generally,antiplatelet agents, anticoagulant agents, antimitotic agents,antioxidants, antimetabolite agents, and anti-inflammatory. Such coatingmay be useful to prevent excessive tissue in-growth. Such electrodes mayfurther include a low-polarization coating such as TiN. Alternatively,the electrodes may be coated with an antibiotic or otherbiologically-active agent used to prevent infections and inflammation.

In another embodiment, the can itself may include a subcutaneouselectrode array of the type described in U.S. Pat. No. 5,331,966, whichis incorporated herein by reference in its entirety. This type of array,which is provided by the Medtronic Model 926 Reveal Plus ImplantableLoop Recorder, includes at least two sensing electrodes on the can forsensing of cardiac signals. In all such systems, it will be understoodthat the electrodes A, B, C on the surface of the housing areelectrically isolated from one another and the conductive surface of thepulse generator housing 10 through suitable insulating bands andelectrical feedthroughs as described in U.S. Pat. No. 4,310,000,incorporated herein by reference. Examples of possible electrodeorientations and configurations of a three electrode system comprisingthe electrodes are set forth in FIGS. 4A through 4G.

FIG. 4A is a side view of a pulse generator illustrating the orientationof orthogonally-disposed electrodes A, B and C with two electrodes onthe connector block 418 and one electrode on the pulse generator case410. The spacing of the electrodes A, B and C on each of the illustratedorientations of FIG. 4A through 4G may be on the order of about one inchbut can be larger or smaller depending on the exact size of the device.Smaller devices and closer spacing will require greater amplification.

FIG. 4B is a side view of a pulse generator wherein at least one of theelectrodes extends away from the pulse generator by a lead extensionmember 420 to achieve a greater inter-electrode spacing, if desirable.

FIG. 4C is a side view of a pulse generator wherein at least one of theelectrodes 230 is located at a proximal end of a lead 432, which may bea lead coupled at a distal end to a subcutaneous electrode or electrodearray.

FIG. 4D is a side view of a pulse generator wherein multiple electrodesare located of an edge of a device housing. It will be understood thatthe electrodes placed on the edge of the pulse generator case couldconstitute insulated pins of feedthroughs extending through the wall ofthe case. As illustrated in FIGS. 4C and 4D, the relative orientation ofthe electrodes may vary somewhat from the orthogonal orientationdepicted in FIGS. 4A and 4B.

FIG. 4E is a side view of yet another embodiment of a device housingincluding an array of electrodes.

FIG. 4F is a side view of a device having a first alternative “T” shape.This shape allows at least two of the electrodes A and C to bepositioned at a maximum distance from one another, optimizing signalreception between the two electrodes.

FIG. 4G is a side view of a device having a second alternative“boomerang” shape which may be used to optimize electrode positioning sothat better signal reception is achieved.

It will be appreciated that the shapes, sizes, and electrodeconfigurations of the devices shown in FIGS. 4A through 4G are exemplaryonly, and any other shape, size or electrode configuration imaginable iswithin the scope of the current invention. As will be appreciated bythose skilled in the art, those configurations allowing for greaterinter-electrode distances will generally provide better signalreception. As such, it is usually desirable to provide electrodes on atleast two quadrants of the device.

As described above, in one embodiment, the current invention provides apulse generator coupled to one or more subcutaneous electrodes orelectrode arrays. The electrodes provide electrical stimulation to apatient based on sensed cardiac signals. The sensed signals may beobtained using a selected pair of sensing electrodes, which may resideon one or more of the leads coupled to pulse generator 10, or on thedevice housing itself, as indicated by FIGS. 4A through 4G.

Although all of the foregoing examples illustrate a housing includingthree electrodes, more than three electrodes may be provided. In oneembodiment, four or more electrodes may be coupled or adjacent to thedevice, and the physician may select which of the electrodes will beactivated for a given patient. In one embodiment, cardiac signals aresensed between a selected pair of the electrodes based on a signaloptimization method. One embodiment of this type of method is disclosedin U.S. patent application Ser. No. 09/721,275 filed Nov. 22, 2000, nowU.S. Pat. No. 6,505,067, and incorporated herein by reference in itsentirety.

Regardless of which one or more electrodes or electrode pairs areselected for monitoring purposes, the sensed cardiac signals may beanalyzed to detect the presence of an arrhythmia. The arrhythmiadetection system and method could be, for example, that employed by theMedtronic Model 9526 Reveal Plus device commercially available fromMedtronic Corporation. Alternatively, a detection method such asdescribed in U.S. Pat. No. 5,354,316 or 5,730,142 could be employed. Ifan arrhythmia is detected, appropriate therapy may be administered. Asdescribed above, one embodiment of the invention includes at least onesubcutaneous defibrillation electrode array. If monitoring indicates thepresence of a tachyarrhythmia or ventricular fibrillation, ahigh-voltage shock may be delivered between one or more subcutaneousdefibrillation electrode(s) and a shocking surface of the can, or one ormore electrodes on the can. The shock may alternatively be deliveredbetween multiple defibrillation electrodes. The monitoring system wouldthen determine whether the arrhythmia or fibrillation has terminated. Ifnot, another shock will be administered. This therapy will continueuntil normal rhythm has been restored. In one embodiment, signalsindicative of sensed cardiac waveforms may be stored in RAM 104 andlater transferred to an external device via a communication system suchas telemetry circuitry 120.

According to another aspect of the invention, the sensing electrodes maybe placed on a surface of the can that is different from the shockingsurface of the can. Preferably, the shocking surface is adjacent tomuscle tissue, whereas the sensing electrodes are placed adjacent tosubcutaneous tissue.

As described above, therapy for bradyarrhythmia may be provided inaddition to, or instead of, the tachyarrhythmia therapy. In thisembodiment, output circuit 108 includes the capability to deliverlower-voltage pulses for transthoracic pacing therapy forbradyarrhythmias, as described above in reference to FIG. 1. Theselower-voltage pulses could be on the order of between 50 and 150 volts,for example. In one embodiment, these pulses have an amplitude of around100 volts. Monitoring for a bradyarrhythmia could be accomplished usingthe sensing electrodes discussed above. For example, the device may beprogrammed to detect a period of asystole that is greater than apredetermined period, such as three seconds. When a period greater thanthis length is detected, the output circuit of the device is charged tothe pacing voltage. A transthoracic, monophasic pacing pulse may then bedelivered between the shocking surface of the can and a subcutaneouselectrode or electrode array, or between two such electrode or electrodearrays. The sensing electrodes monitor the cardiac waveform to ensurethat the pacing pulse is only delivered during predetermined periods ofthe cardiac cycle. For example, delivery of the pulse should not occurduring the occurrence of a T-wave.

Following delivery of a pacing pulse, the output circuit begins chargingin preparation for delivery of another pulse while monitoring of thecardiac signals continues. For example, monitoring of the patient'sheart rate may be performed to determine whether it is less than somepredetermined rate such as forty beats per minute. If so, anothertransthoracic, monophasic pacing pulse is delivered. This process ofpulse delivery followed by charging of the output circuit is repeateduntil an intrinsic heart rate of greater than the predetermined minimumrate is detected.

The transthoracic pacing provided by the current invention will likelybe uncomfortable for the patient. Thus, this function is not intended toprovide chronic therapy. Once therapy delivery has occurred for abradyarrhythmic episode, a more traditional device should be implantedto provide long-term therapy. In one embodiment, the device may recordwhether any ACC/AHA class I pacing indications has been met by thedetected bradyarrhythmic event. For example, if asystole greater thanthree seconds and/or an escape rate less than forty beats per minute hasbeen detected, these indications are recorded. This data may then betransferred to an external device to generate a physician notification.Other actions may be taken, such as sounding an alarm, for example.

FIG. 5 is a timing diagram illustrating one embodiment of a detectionmethod used during bradyarrhythmia monitoring. If asystole is detectedfor greater than, or equal to, a first predetermined, time period 500such as three seconds, charging of output capacitors occurs to apredetermined voltage such as 100 volts. This charging occurs duringtime period 502. At time 504, a first pacing pulse is delivered, andrecharging of the capacitors begins at time 506. Monitoring for anescape rate longer than a predetermined rate occurs during time period508, which in one embodiment is 1500 milliseconds. Thereafter, a secondpacing pulse is delivered at time 510 if an intrinsic beat does notoccur. At time 512, recharging occurs, and monitoring for the escaperate again proceeds. If such therapy is not discontinued because of there-occurrence of the patient's intrinsic normal heart beat, the patientwill be required to seek immediate emergency attention, since suchtherapy will be uncomfortable for the patient. The times utilized toprovide therapy as shown in FIG. 5 may be programmable.

It may be appreciated from the foregoing discussion that providingrepeated therapy, and in particular, repeated high-voltage pacingstimulation, will deplete a system power source, such as a battery,relatively quickly. Therefore, in one embodiment, the power source isrechargeable. For example, the pulse generator may include rechargeablenickel cadmium batteries. Such batteries may be recharged over a periodof several hours using a radio frequency link. Alternatively, arechargeable capacitive energy source such as disclosed in U.S. Pat. No.4,408,607 to Maurer may be utilized. In yet another embodiment, thepulse generator may include both an implanted radio frequency (RF)receiving unit (receiver) incorporating a back-up rechargeable powersupply and a non-rechargeable battery, as described in U.S. Pat. No.5,733,313 incorporated herein by reference. The rechargeable powersupply is charged by an external RF transmitting unit worn by thepatient. Any other type of rechargeable power supply known in the artfor use with implantable medical devices may be used in the alternative.

In one embodiment, the power source selected for use in the currentinvention is capable of delivering up to ten therapy shocks, withadditional power being available for threshold testing. However,compromises will exist since the power source capacity will determinedevice size. In yet another embodiment the device is a 75-joule devicehaving a volume of no more than 75 cubic centimeters. Preferably, thedevice includes a power source and associated charge circuitry thatprovides a charge time of no more than three minutes during the usefullife of the device. In another embodiment, the device should be capableof delivering a 35-joule shock after a one-minute charge time over theuseful life of the device.

FIGS. 6 through 9 illustrate various exemplary electrode configurationsas may be used with the current invention.

FIG. 6 is a block diagram illustrating an electrode array 300 positionedaround a patient's side, with fingers 304 extending to the patient'sback. Electrical stimulation is delivered between the electrode arrayand the device can 10, which is positioned over the left ventricle. Inone embodiment, sensing electrodes 600 are positioned substantiallyfacing toward subcutaneous tissue.

FIG. 7 is a block diagram illustrating an electrode array positioned ona patient's back in a more superior position than is shown in FIG. 6.Electrical stimulation is delivered between the electrode array and thedevice can 10, which is positioned in the abdominal cavity.

FIG. 8 is a block diagram illustrating an electrode array positionedaround a patient's side, with fingers 304 extending to the patient'sback in a more posterior position than is shown in FIG. 6 or 7.Electrical stimulation is delivered between the electrode array and thedevice can, which is positioned proximal the right-side of the heart.

FIG. 9 is a block diagram illustrating an electrode array with fingers304 positioned on a patient's back, and a second subcutaneous diskelectrode 306 such as electrode 16 (FIG. 1) positioned on a patient'schest. Electrical stimulation may be delivered from one of electrodes304 or 306 to the other electrode and/or the device housing 10.Alternatively, stimulation may be provided from both electrodeassemblies to the device housing. In yet another embodiment, one or moreadditional subcutaneous electrode or electrode arrays may be coupled tothe device for providing high-voltage shocks, for sensing cardiacsignals, and/or for delivering SCS, TENs, or subcutaneous low-voltagestimulation as discussed above. If desired, the device may includeprogrammable logic to selectably enable those electrode and/or electrodearrays to be activated during a given therapy delivery session. Forexample, switching networks may be incorporated into output circuitry108 and/or input circuitry 110 (FIG. 2) such that this type ofprogrammably selected therapy may be provided. In one instance, it maybe desirable to activate one electrode configuration to optimize sensingof cardiac signals, while utilizing another configuration to provideoptimal therapy delivery.

The above-described inventive system and method provides a therapy thatavoids the risks of transvenous lead delivery. Such a system may be usedfor patients that are at-risk for arrhythmias, but have not yetexperienced a confirmed arrhythmic episode. The device may thereforeprovide a needed long-term monitoring function, as well as anyinterventional therapy that is required. Preferably, after an episode isdetected and therapy is delivered for a first time, the current systemwould be replaced with a more conventional implantable defibrillator.

As discussed above, the inventive system provides many importantbenefits over other conventional systems for some patients. Theprocedure is faster because there is no need for venous or epicardialaccess, and therefore the procedure is less invasive, and would notrequire procedures needing sophisticated surgical facilities anddevices. Additionally, the implant procedure can be accomplished withoutexposing the patient to potentially-harmful radiation that accompaniesfluoroscopy. The risk of infection is reduced, and the procedure may beprovided to patients that are contraindicated for a more traditionaldevice. Additionally, one hundred percent patient compliance isachieved, and the system is more comfortable than externally-worndevices. The system is well suited for pediatric use, since theplacement of the electrodes allows lead length to be easily extended asa patient grows. The system may also be employed in parts of the worldwhere more long-term therapies and treatments are not available, andwhere sophisticated surgical skills and equipment cannot be readilyobtained.

What is claimed is:
 1. A method for treating patient arrhythmias,including the methods of: a) providing a subcutaneous pulse generator ina canister having a conductive surface; b) providing a monitoringcircuit in the canister to monitor the patient's cardiac signals forarrhythmias; and c) providing a subcutaneous electrode array to deliverelectrical therapy to a patient without a transvenous, intracardiac, orepicardial electrode.
 2. The method of claim 2, wherein the monitoringcircuit includes a circuit to sense a bradyarrhythmia event, and whereinthe subcutaneous electrode array is configured to deliver at least onepacing pulse to the patient upon detection of the bradyarrhythmia event.3. A method of implanting a subcutaneous cardioverter-defibrillator in apatient for treating arrythmias without a transvenous, intracardiac orepicardial electrode, comprising: implanting an electrode subcutaneouslyin the patient; and placing an electrically active canistersubcutaneously in the patient, wherein the canister contains a source ofelectrical energy and operational circuitry that senses the presence ofpotentially fatal heart rhythms and has circuitry for deliveringelectrical cardioversion-defibrillation energy between the canister andthe electrode, and wherein the canister is electrically connected to theelectrode.
 4. The method of claim 3, wherein the electrode can besubcutaneously implanted at various locations in the body.
 5. The methodof claim 3, wherein the canister can be subcutaneously implanted atvarious locations in the body.
 6. The method of claim 3, wherein theoperational circuitry comprises an impedance detection means formeasuring impedance.
 7. The method of claim 3, wherein the operationalcircuitry can measure cardiac output.
 8. The method of claim 3, whereinthe steps of implanting an electrode subcutaneously in the patient andplacing an electrically active canister subcutaneously in the patientare performed without a transvenous, intracardiac or epicardialelectrode being implanted.
 9. The method of claim 3, wherein the stepsof implanting an electrode subcutaneously in the patient and placing anelectrically active canister subcutaneously in the patient are performedwithout venous or epicardial access.
 10. A method for providinganti-arrhythmia therapy via a subcutaneous cardioverter-defibrillatorand without a transvenous, intracardiac or epicardial electrode,comprising: implanting a canister subcutaneously in a patient, thehousing enclosing and containing cardioversion-defibrillation circuitryand defining at least one electrically conductive electrically connectedto the cardioversion-defibrillation circuitry; implanting a leadsubcutaneously, the lead having a lead electrode formed on a distal endthereof and configured to connect to the canister to electricallyinterface, the lead electrode to the cardioversion-defibrillationcircuitry; and delivering an electrical therapy comprising ananti-arrhythmia waveform from the lead electrode to the at least oneelectrically conductive surface.
 11. A method according to claim 10,further comprising: providing a plurality of sensing electrodes on thelead, each sensing electrode interfacing with sensing circuitry in thecardioversion-defibrillation circuitry; and monitoring and derivingcardiac physiological measures via the sensing electrodes.
 12. Themethod of claim 10, wherein the steps of implanting a canister andimplanting a lead are performed without a transvenous, intracardiac orepicardial electrode being implanted.
 13. The method of claim 10,wherein the steps of implanting a canister and implanting a lead areperformed without venous or epicardial access.
 14. A method ofimplanting a subcutaneous cardioverter-defibrillator in a patient fortreating arrhythmias without a transvenous, intracardiac or epicardialelectrode, comprising: implanting an electrode subcutaneously in thepatient; placing an electrically active canister subcutaneously in thepatient, wherein the canister contains a source of electrical energy andoperational circuitry that senses the presence of potentially fatalheart rhythms and has circuitry for delivering electricalcardioversion-defibrillation energy between the canister and theelectrode, and wherein the canister is electrically connected to theelectrode; wherein the operational circuitry can detect the presence ofbradycardia rhythm and the canister includes circuitry for deliveringelectrical cardiac pacing energy when the operational circuitry sensesthe bradycardia rhythm.
 15. The method of claim 14, wherein theelectrode can be subcutaneously implanted at various locations in thebody.
 16. The method of claim 14, wherein the canister can besubcutaneously implanted at various locations in the body.
 17. Themethod of claim 14, wherein the operational circuitry comprises animpedance detection means for measuring impedance.
 18. The method ofclaim 14, wherein the operational circuitry can measure cardiac output.19. The method of claim 14, wherein the steps of implanting an electrodesubcutaneously in the patient and placing an electrically activecanister subcutaneously in the patient are performed without atransvenous, intracardiac or epicardial electrode being implanted. 20.The method of claim 14, wherein the steps of implanting an electrodesubcutaneously in the patient and placing an electrically activecanister subcutaneously in the patient are performed without venous orepicardial access.